Recently, electrochemical cells have actively been studied. Of these electrochemical cells, as an example a fuel cell includes a system which generates electricity by an electrochemical reaction between fuel such as hydrogen and an oxidizing agent such as oxygen. In particular, a PEFC (Polymer Electrolyte Fuel Cell) has been put into practical use as a domestic stationary power supply or automobile power supply because the influence on the environment is small. This PEFC generates water as a reaction product, and can operate at a temperature lower than those of other fuel cells. One big objective for the spread of PEFC is cost reduction by reducing the amount of use of noble metal catalyst.
As a catalyst layer included in each electrode of the PEFC, a carbon-supported catalyst obtained by supporting a catalyst material by a carbon black support is generally used.
When the PEFC is used as, e.g., an automobile power supply, the carbon support included in the catalyst layer on the cathode side becomes corroded by start and stop, and the catalyst itself supported by the carbon support also dissolves. It has been reported that this accelerates deterioration of the catalyst layer and an MEA (Membrane Electrode Assembly) including the catalyst layer. In order to secure sufficient durability, adopting of a carbonless catalyst layer formed by sputtering or vapor-depositing a large amount of catalyst has been considered. An example is a catalyst layer obtained by sputtering platinum on a whisker substrate.
By being carbonless, deterioration of the catalyst support due to corrosion can be avoided, and the amount of noble metal catalyst used can be reduced. However, the properties of such catalyst layers become unstable when a highly humid fuel is supplied. When such a catalyst layer is applied to the cathode side of a fuel cell, water generated by the electrode reaction causes a water flooding phenomenon, and property deterioration occurring near room temperature is particularly significant. Accordingly, demands have arisen for a high-durability catalyst layer having good humidity robustness (i.e., a catalyst layer favorable in terms of humidity dependence). Since carbon is hydrophobic, introducing carbon to the noble metal catalyst layer is advantageous from the viewpoint of water handling. Methods such as introducing fibrous carbon between stacked platinum sheets have been proposed. However, properties when platinum amount is little are still unsatisfactory and needs further improvement.